This invention relates to data measuring of downhole conditions within wells during drilling and more particularly relates to apparatus and methods for telemetering data in such operations using an acoustic signal transmitted through the drilling fluid during drilling.
Various logging-while-drilling techniques for telemetering data representing downhole conditions during drilling of a well have been suggested. One approach uses a technique which imparts an acoustic signal, modulated according to the sensed conditions, to the drilling fluid, i.e., the drilling mud, for transmission to the entrance of the well where it is received and decoded by uphole electronics circuitry. This basic technique is described in detail in U.S. Pat. No. 3,309,656, issued Mar. 4, 1967 to Godbey entitled "Logging-While-Drilling System." In this system the modulated signal is applied to the drilling fluid using an acoustic signal generator which includes a movable member for selectively interrupting the drilling fluid. At least part of the flow of the drilling fluid is through the acoustic generator, and the movable member selectively impedes this flow, transmitting a continuous acoustic wave uphole within the drilling fluid.
The acoustic signal is preferably phase shift keyed modulated, as disclosed in U.S. Pat. No. 3,789,355, issued Jan. 29, 1974, to Patton entitled "Method and Apparatus For Logging While Drilling." According to phase shift keyed (PSK) modulation, the data derived in response to the sensed downhole condition is initially encoded into binary format, and the acoustic signal generator is driven at speeds so that the phase of a carrier wave generated in the drilling fluid is indicative of the data. In particular, a non-return to zero type PSK mode is used wherein the phase of the carrier signal is changed only upon each receipt of data of a predetermined value. For example, for data encoded in binary, the phase of the carrier wave may be changed for each occurring of a logic 1 data bit.
Ideally the phase change of the carrier signal would be instantaneous upon occurrence of the data of the particular value. This is because the downhole telemetering unit is continuously transmitting data to the uphole receiving instruments where the data in turn is continuously decoded. Any delays in effecting the phase change and in returning the acoustic signal to its carrier frequency introduce errors and/or inefficiencies into the system.
As a practical matter, however, the phase of the acoustic signal cannot be changed instantaneously in response to data of the predetermined value. Inherent delays are introduced by the physics of the system. The motor control circuitry which operates the motor-driven acoustic generator is adjusted accordingly to effect optimum response of the generator. Past proposals, such as the above-referenced Godbey and Patton patent, and in U.S. Pat. No. 3,820,063, issued June 25, 1974, to Sexton et al. entitled "Logging While Drilling Encoder," have proposed an analog implementation of the motor control circuitry. Because the motor control circuitry operates at a relatively low frequency, the analog approach has resulted in a system which may operate at a less than optimum data encoding/decoding rate. Furthermore, such analog circuitry suffers from the inherent disadvantages of instability over wide ranges of temperature, resulting in a less than optimally dependable system. More specifically, normal temperatures encountered within a borehole during drilling varies from 25.degree. C to greater than 175.degree. C, causing inherent changes in the device characteristics of the analog circuitry.
Furthermore, the analog approach suffers due to the rugged environment encountered during drilling conditions. The extreme vibrations and shock received by the analog circuitry not only reduces its longevity but also tends to render the circuitry out of adjustment.
Still further, analog circuits are relatively expensive to manufacture and test.